1. Field of the Invention
This invention relates to detection and interruption of currents in an AC electrical power circuit experiencing arcing faults. More particularly, it relates to apparatus with increased sensitivity to difficult to distinguish arcs, such as carbon arcs, yet with enhanced immunity to nuisance tripping.
2. Background Information
Arc faults can occur in electrical systems, for instance between adjacent paired conductors, between exposed ends of broken conductors, at a faulty connection, where carbon deposits have collected adjacent terminals or outlets, and in other situations where conducting elements are in close proximity. Arc faults in AC systems can be intermittent; however, arcs caused by carbon deposits can occur regularly in successive half-cycles.
Arc faults typically have high resistance, so that the arc current is below the instantaneous or magnetic trip threshold of conventional circuit breakers. Also, the intermittent nature of an arc fault can create an average RMS current value which is below the thermal threshold for such circuit breakers. Even so, the arcs can cause damage or start a fire if they occur near combustible materials. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads which draw similar currents and which would, therefore, cause nuisance trips.
Much attention has been directed to trying to distinguish arcing currents from other intermittent currents. It has been recognized that arcing faults generate a great deal of high frequency noise, and flirter, that there are periods of quiescence in the high frequency component. Some circuit breakers look to such features to differentiate arcing faults from other phenomena. Circuit breakers which rely upon such detailed characteristics of current waveform to detect arc faults typically utilize a microprocessor to perform the analysis. They also require fairly good quality analog-to-digital converters to capture the high frequencies of interest. Thus, such arc fault detectors add significantly to the cost of a circuit breaker, and in die case of the typical residential circuit breaker, can multiply its cost many times. Yet even such sophisticated circuit breakers are subject to nuisance trips when confronted with some common load devices.
It has also been recognized that arc faults generate a step increase in current when the arc is struck. However, many typical loads generate a similar step increase when a device is turned on. In many cases, the step increase produced by a load is a singular event and can be distinguished from an arc fault which generates repetitive step increases by counting step increases during an interval such as a few half-cycles. A more sophisticated variant of this type of arc fault detector maintains a time attenuated accumulation of step increases and generates a trip when a selected level of the accumulation is reached. This type of detector provides a faster trip on large step increases while reducing nuisance trips.
A dimmer circuit provides unique problems for an arc fault detector which responds to the step increases generated by the striking of an arc. A dimmer, when phased back, produces a pattern of step increases in current each half-cycle. Under steady state conditions, the amplitude of these pulses will be below that of an arc current, and can, therefore, be distinguished on that basis. However, if the dimmer is used to control a tungsten bulb, the cold filament has a very low resistance on start-up which produces a large initial pulse with subsequent pulses decaying in amplitude as the filament rapidly warms up. This characteristic of a dimmer has also been used to distinguish it from arc faults, but it still has been necessary to maintain die threshold for arc detection above the handle rating to avoid nuisance tripping on a dimmer.
As mentioned, carbon arc faults can strike in successive half-cycles and thus look very similar to a dimmer. This presents a difficult challenge in meeting the code requirement for a reliable response to carbon arcs within eight half-cycles of onset while rejecting nuisance trips in response to a dimmer including turn on of a cold tungsten bulb.
It is an object of the invention, therefore, to provide an improved arc fault detector and circuit breaker incorporating the same which has increased sensitivity to arc faults, while also having enhanced immunity to nuisance trips.
It is a particular object of the invention to provide such apparatus which responds reliably to carbon arcs while rejecting trips attributable to dimmers and especially dimmers controlling tungsten bulbs.
It is another object of the invention to increase the sensitivity of an arc fault detector which responds to step increases in current caused by the striking of an arc.
It is another object of the invention to provide such apparatus which is economical and compact and can be implemented in the limited space available in small circuit breakers.
These objects and others are satisfied by the invention which is directed to an arc fault detector, and a circuit breaker incorporating such an arc fault detector, having a current detector which includes an average instantaneous current generator which generates a running average of the instantaneous current. Averaging over the fundamental period of the ac current results in a signal that can be used to distinguish arcing faults over normal loads. The average over die fundamental period of a pure sine wave is ideally zero. The magnitude of the average current over the period of the fundamental is a function of the non-repeatability of the current waveform from half-cycle to half-cycle. This function produces greater outputs for current waveforms that vary from half-cycle to half-cycle in magnitude and pulse width. Arcing faults, being random in nature and having varying magnitudes, pulse widths and missing half-cycles, produce magnitudes greater than current waveforms of a consistent half-cycle to half-cycle nature for normal loads. The arc fault detector of the basic form of the invention includes a trip signal generator which generates a trip signal when die average instantaneous current reaches a threshold value.
As mentioned, die carbon arc can have a very consistent half-cycle to halfcycle current waveform, and therefore, it does not produce a sufficient magnitude of average current to distinguish it as an arc. Accordingly, a second embodiment of the invention incorporates additional features which ensure detection of the carbon arc. An approximation of the average instantaneous current over the fundamental period can be obtained with an analog bandpass filter having a center frequency which is below the fundamental frequency of the ac current. This approximation includes a fraction of the fundamental magnitude. Using this approximation, the carbon arc produces sufficient magnitudes to be effectively distinguished as an arc, while maintaining the increased sensitivity to random arcing and immunity to nuisance tripping. Tis also causes the output of the fundamental to no longer be zero. The threshold for this value can be set so that the carbon arc can be distinguished from the steady state dimmer current which will have a lower fundamental amplitude.
The arc fault detector must also be able to ignore inrush currents such as those associated with the motor start-up. Hence, in another embodiment of the invention, a di/dt or pulse detector is added to determine if a step increase in current has occurred within each half-cycle. In this embodiment, the trip signal generator responds only to a function of pulses generated in half-cycles in which the average instantaneous current is more than the threshold. Preferably, the trip signal generator generates the trip signal as a time attenuated accumulation of pulses in half-cycles in which the average instantaneous current is more than the threshold. This pulse generator can be implemented as a second bandpass filter having a center frequency above the fundamental frequency so that it generates a pulse in response to a step increase in the current. This bandpass filter can be set to trip at a desired pure 60 Hz current waveform by adjusting the level of 60 Hz in the di/dt bandpass filter. This serves as an electronic overcurrent detector. Thus, this circuit consists of two bandpass channels, one below the fundamental and one above the fundamental. The trip signal generator logically ANDs the output to these two bandpass filters.
To further improve sensitivity to arcs and to increase immunity to nuisance tripping, the pulse generator can be configured to generate an output as a function of a predetermined value of pulses generated each time an arc is struck. Preferably, this function is a time attenuated accumulation of the pulses. When a predetermined level of this time attenuated accumulation is reached, the pulse generator generates an output which enables the time attenuated accumulation of the average instantaneous current. If a sufficient average instantaneous current occurs, when enabled, a trip signal is generated.
The invention also includes incorporation of this novel arc fault detector in a circuit breaker which interrupts current in response to detection of an arc fault.
Accordingly, it is an object of the invention to provide an arc fault detector and a circuit breaker incorporating such an arc fault detector with improved sensitivity to arc faults, including carbon arc faults, and with enhanced immunity to nuisance trips, including response to dimmers and especially dimmers used to control tungsten bulbs.
It is also an object of the invention to provide such apparatus which monitors the average instantaneous current for use in detecting arc faults.
It is a further object of the invention to provide an enhanced version which combines monitoring of average instantaneous current with detection of step increases in current such as are produced each time an arc is struck.
It is an overall object of the invention to provide such apparatus which is economical and practical to implement.